This invention relates to fire resistant polycarbonate compositions and more particularly transparent, fire resistant polycarbonate compositions.
Polycarbonate is widely used in the manufacture of molded parts because of its excellent impact resistance, high heat distortion temperature, and transparency. Flame-retardant grades of polycarbonate have been developed for applications such as housings for electronic devices, where safety considerations require certain levels of flame-resistance. These flame-retardant polycarbonates are typically prepared using bromine- or chlorine-containing flame-retardants. Recently, concerns about environmental contamination from the bromine- or chlorine-containing flame-retardants has created a need for transparent polycarbonates that contain little or no bromine or chlorine. Moreover, as molded parts become larger and contain more plastic, there is a need for flame-retardant grades that can pass more stringent flame-retardant testing, such as that set out in the Underwriter""s Laboratory 5V A test protocol.
Siloxane additives have been used to improve the flame retardancy of polycarbonate as taught by U.S. Pat. Nos. 5,449,710 and 6,184,312, but may create certain performance issues in certain applications. Siloxane additives may migrate to the surface of molded parts and affect their performance. They may volatize from the parts and affect the performance of nearby electrical components. In a different embodiment, U.S. Pat. Nos. 3,189,662; 3,419,635; 4,732,949 and 5,068,302 disclose a variety of polycarbonate/siloxane copolymer structures that overcome the problems associated with siloxane additives. In yet another teaching, U.S. Pat. No. 6,072,011 discloses a polycarbonate/siloxane copolymer based on 4-8 wt. % of a eugenol-capped linear siloxane with block lengths of 45-55 dimethylsiloxane units. This material provides excellent hydrolytic stability, low temperature ductility, and 5V flame-retardant performance at 3 mm thickness, but the material is not transparent.
Applicants have surprisingly found that by dramatically lowering the amount of short-chained eugenol-capped siloxanes, one obtains a transparent branched polysiloxane-polycarbonate block copolymer that meets the more stringent UL 5V ratings, and which is essentially free of volatile siloxanes and bromine or chlorine-containing flame retardants.
The invention relates to a transparent branched polysiloxane-polycarbonate block copolymer composition comprising less than 1 wt. % of an eugenol-capped siloxane having block lengths of 5-15 dimethyl siloxane units, wherein the polycarbonate composition has a UL94 5V rating at thickness greater than or equal to about 4 millimeters.
The invention also relates to a process for preparing a branched polysiloxane-polycarbonate block copolymer composition comprising less than 1 wt. % of an eugenol-capped siloxane having block lengths of 5-15 dimethyl siloxane units.
Transparent is herein defined as having a percent transmission of about 85 or greater and a haze value of about 5 or less when measured according to ASTM D1003, which is incorporated herein by reference, at a thickness of 3.2 mm.
Polydiorganosiloxane/polycarbonate block copolymers are well-known in the art, see for example U.S. Pat. Nos. 3,189,662; 3,419,634; 4,732,949; and 5,068,302 which are incorporated here by reference disclosing resins comprising polycarbonate and polysiloxane blocks and processes for preparing polycarbonate-polysiloxane block copolymers.
Polycarbonate Block.
The polycarbonate blocks in the composition of the present invention comprise recurring units of formula I: 
wherein Ra and Rb each represent a monovalent hydrocarbon group and may be the same or different; p and q are each independently integers from 0 to 4; and Xa represents one of the groups of formula: 
wherein Rc and Rd each independently represent a hydrogen atom or a monovalent linear or cyclic hydrocarbon group and Re is a divalent hydrocarbon group.
The polycarbonate blocks may be prepared by reacting a dihydroxy compound such as bisphenol with a carbonate precursor such as phosgene, a haloformate, a carbonate or a carbonate ester, generally in the presence of an acid acceptor and a molecular weight regulator. It is also possible that the block copolymer may be made by other methods known in the art for making polycarbonates such as transesterification as disclosed in U.S. Pat. No. 3,154,008.
Useful polymerization methods include interfacial polymerization, melt polymerization, and redistribution.
As used herein, the term xe2x80x9cdihydroxy compoundxe2x80x9d includes, for example, bisphenol compounds of formula II: 
wherein Ra and Rb each represent a monovalent hydrocarbon group and may be the same or different; p and q are each independently integers from 0 to 4; and Xa represents one of the groups of formula: 
wherein Rc and Rd each independently represent a hydrogen atom or a monovalent linear or cyclic hydrocarbon group and Re is a divalent hydrocarbon group.
Some illustrative, non-limiting examples of suitable dihydroxy compounds include the dihydroxy-substituted aromatic hydrocarbons disclosed by name or formula (generic or specific) in U.S. Pat. No. 4,217,438, which is incorporated herein by reference. A nonexclusive list of specific examples of the types of bisphenol compounds that may be represented by formula (II) includes: 1,1-bis(4-hydroxyphenyl) methane; 1,1-bis(4-hydroxyphenyl) ethane; 2,2-bis(4-hydroxyphenyl) propane (hereinafter xe2x80x9cbisphenol Axe2x80x9d or xe2x80x9cBPAxe2x80x9d); 2,2-bis(4-hydroxyphenyl) butane; 2,2-bis(4-hydroxyphenyl) octane; 1,1-bis(4-hydroxyphenyl) propane; 1,1-bis(4-hydroxyphenyl) n-butane; bis(4-hydroxyphenyl) phenylmethane; 2,2-bis(4-hydroxy-1-methylphenyl) propane; 1,1-bis(4-hydroxy-t-butylphenyl) propane; bis(hydroxyaryl) alkanes such as 2,2-bis(4-hydroxyphenyl) propane; 1,1-bis(4-hydroxyphenyl) cyclopentane; and bis(hydroxyaryl) cycloalkanes such as 1,1-bis(4-hydroxyphenyl) cyclohexane.
It is also possible to employ two or more different dihydroxy compounds or copolymers of a dihydroxy compound with a glycol or with a hydroxy- or acid-terminated polyester or with a dibasic acid or hydroxy acid in the event a carbonate copolymer rather than a homopolymer is desired for use. Polyarylates and polyester-carbonate resins or their blends can also be employed.
Siloxane Block.
The siloxane blocks for use in preparing the branched copolymer of the invention may be characterized as bisphenolsiloxanes. The preparation of these bisphenolsiloxanes is accomplished by the addition of a polydiorganosiloxane to a phenol containing an alkenyl substituent, according to the schematic formula: 
wherein R1 and R2 are each independently hydrocarbyl and where D is an integer of from about 5 to about 20. In one embodiment, R1 is methyl and R2 is methyl or phenyl, and D is about 5-15.
The essential features of the process to prepare the bisphenolsiloxanes are described by Vaughn, U.S. Pat. No. 3,419,635 (December 1968). For instance, the process is exemplified in example 8 of this Vaughn patent which describes the addition of a hydrogen-terminated polydimethylsiloxane to an allylphenol in the presence of a catalytic amount of chloroplatinic acid-alcohol complex at 90-115.degree. C.
In one embodiment, the polysiloxane blocks are made from bisphenolpolysiloxanes where R1 and R2 are methyl, and where Y is methoxy located ortho to the phenolic substituent. These are readily prepared by addition of a hydrogen-terminated polysiloxane to two molar equivalents of eugenol (4-allyl-2-methoxyphenol) in a reaction advantageously catalyzed by platinum or its compounds.
The allylphenols in the schematic formula above are also well known compounds, described along with methods for their preparation, by Tarbell, Chemical Reviews 27, 495ff (1940). In one embodiment, the allylphenol is eugenol, 4-allyl-2-methoxyphenol, since it is readily available as a synthetic or as a natural product and affords a bisphenolpolysiloxane of favorable reactivity.
The polysiloxane blocks and optional salt based flame retardant(s) are present in quantities effective to achieve a UL94 5V rating at 4 mm. In one embodiment, the polysiloxane blocks are present in an amount of less than 1 wt. %. In a second embodiment, the amount is about 0.25 to 0.75 wt. %, based on the total resin weight. In yet a third embodiment, the amount is about 0.5 wt. %.
Branching Agents.
The branched copolymers of the present invention are prepared by adding a branching agent during polymerization. Branching agents are well-known in the art and may comprise polyfunctional organic compounds containing at least three functional groups which may be hydroxyl, carboxyl, carboxylic anhydride, carboxylic acid chloride and mixtures thereof.
Specific examples include trimellitic acid, trimellitic anhydride, trimellitic trichloride, tris-p-hydroxy phenyl ethane (xe2x80x9cTHPExe2x80x9d), isatin-bis-phenol, tris-phenol TC (1,3,5-tris((p-hydroxyphenyl)isopropyl)benzene), tris-phenol PA (4(4(1,1-bis(p-hydroxyphenyl)-ethyl) alpha,alpha-dimethyl benzyl)phenol, trimesic acid and benzophenone tetracarboxylic acid.
The branching agents may be added at a level of about 0.05-2.0 weight percent.
Optional Flame Retardants.
The composition of the present invention may further comprise non-volatile siloxane flame retardants salt based flame retardants including alkali metal or alkaline earth metal salts of inorganic protonic acids as well as organic Brxc3x6nsted acids comprising at least one carbon atom. In one embodiment, the salt based flame retardants are sulphonates selected from the group consisting of potassium diphenylsulfon-3-sulphonate (KSS), potassium-perfluorobutane-sulphonate (KPFBS) and combinations comprising at least one of the foregoing.
Salt based flame retardants may be used in amounts of about 0.01 wt % to about 1.0 wt % based on the total resin weight. In one embodiment, the salt based flame retardant is KPFBS with the amount of KPFBS being about 0.05 wt % to about 0.12 wt % based on the total resin weight. In a second embodiment wherein the salt based flame retardant is KSS, the amounts are about 0.35 wt % or less based on the total resin weight.
Optional Additives
The composition of the present invention may include various additives ordinarily incorporated in resin compositions of this type. Such additives are, for example, heat stabilizers; antioxidants; light stabilizers; plasticizers; antistatic agents; mold releasing agents; additional resins; blowing agents; and the like.
Such additives are known to the art of plastics compounding, and may include, for example, about 0.01 to about 0.1 part by weight of a heat stabilizer; about 0.01 to about 0.2 part by weight of an antioxidant; about 0.1 to about 0.7 part by weight of a light stabilizer; about 0.5 to about 3 parts by weight of a plasticizer; about 0.1 to about 3 parts by weight of an antistatic agent; and about 0.1 to about 1 part by weight of a mold releasing agent. The above amounts of the additives are based on 100 parts by weight of the resin composition.
Examples of heat stabilizers include triphenyl phosphite, tris-(2,6-dimethylphenyl)phosphite, tris-(2,4-di-t-butyl-phenyl) phosphite, tris-(mixed mono-and di-nonylphenyl)phosphite, dimethylbenzene phosphonate and trimethyl phosphate. Examples of antioxidants include octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, and pentaerythrityl-tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate]. Examples of light stabilizers include 2-(2-hydroxy-5-methylphenyl)benzotriazole, 2-(2-hydroxy-5-tert-octylphenyl)-benzotriazole and 2-hydroxy-4-n-octoxy benzophenone. Examples of plasticizers include dioctyl-4,5-epoxy-hexahydrophthalate, tris-(octoxycarbonylethyl) isocyanurate, tristearin and epoxidized soybean oil. Examples of the antistatic agent include glycerol monostearate, sodium stearyl sulfonate, and sodium dodecylbenzenesulfonate. Examples of mold releasing agents include pentaerythritoltetrastearate, stearyl stearate, beeswax, montan wax and paraffin wax. Combinations of any of the foregoing additives may be used. Such additives may be mixed at a suitable time during the mixing of the components for forming the composition.
Preparation.
The conversion of the bisphenolpolysiloxane, the dihydroxy compound such as bisphenol, and branching agents into the branched polycarbonate/siloxane block copolymers of the invention may be conducted by known processes for making branched polycarbonates, as described in U.S. Pat. Nos. 3,635,895 and 4,001,184 which are incorporated by reference. All types of polycarbonate end groups are contemplated as being within the scope of the present invention.
In one embodiment, the transparent, fire resistant polycarbonate composition may be synthesized by charging a polymerization vessel with the polysiloxane, aromatic or aliphatic dihydroxy compound, organic solvent, water, endcapping agent, and a tertiary amine. The reaction mixture is stirred vigorously, and phosgene is bubbled through the solution while the pH is maintained between about 8 and 11 by addition of aqueous caustic solution.
When the reaction is complete, the organic layer is separated from the aqueous layer, washed with dilute hydrochloric acid and then washed with deionized water. The solvent is removed, for example by steam precipitation or anti-solvent precipitation, and the polymer is dried.
The composition of the present invention and optional additives may be compounded using any known mixing method. In one embodiment, the raw materials are added directly into the feed section of a melt mixing device (such as an extruder) via separate feed systems. In the melt mixing step, the ingredients are typically melt kneaded in a single screw or twin screw extruder, and extruded as pellets.
The compositions of the present invention may be used for any applications wherein their key properties of transparency, heat resistance, toughness, UV stability, and flame retardancy are required. The moldings may be produced by extrusion or injection molding.